From Longman Dictionary of Contemporary Englishfor all I knowfor all I knowNOT KNOWused to emphasize that you do not know something and say that it is not important to you I don’t know where she is. She could have been kidnapped for all I know. → know
Examples from the Corpus
for all I know• It cost millions. It could be billions for all I know.• And they probably have cameras on it for all I know.• During the war for all I know?• He might even be her source, for all I knew.• There were a few people around, polishing and sweeping, and maybe spreading incense for all I knew.• You might be from the newspapers, for all I know.• The real losers will, for all I know, eat dirt.• I opened the window, and for all I know it's still open.• Snow and ice were deadly for all I knew, never having driven on either. 